This invention pertains to animal drug delivery systems. More particularly this invention pertains to a non-degradable implant for the administration of a substantially constant rate of estradiol to a ruminant animal, which implant may be removed from the animal at the end of the drug administration period.
Long et al. U.S. Pat. No. 3,279,996 describes an implant for releasing a drug in the tissues of a living organism comprising the drug enclosed in a capsule formed of silicone rubber. The drug migrates through the silicone rubber wall and is slowly released into the living tissues. A number of biocompatible silicone rubbers are described in the Long et al. patent. When a drug delivery system such as that described in U.S. Pat. No. 3,279,996 is used in an effort to administer estradiol to a ruminant animal a number of problems are encountered. For example, an excess of the drug is generally required in the hollow cavity of the implant. Also, it is difficult to achieve a constant rate of administration of the drug over a long time period such as from 200 to 400 days as would be necessary for the daily administration of estradiol to a growing beef animal.
Katz et al. U.S. Pat. No. 4,096,239 describes an implant pellet containing estradiol or estradiol benzoate which has an inert spherical core and a uniform coating comprising a carrier and the drug. The coating containing the drug must be both biocompatible and biosoluble, i.e., the coating must dissolve in the body fluids which act upon the pellet when it is implanted in the body. The rate at which the coating dissolves determines the rate at which the drug is released. Representative carriers for use in the coating material include cholesterol, solid polyethylene glycols, high molecular weight fatty acids and alcohols, biosoluble waxes, cellulose derivatives and solid polyvinyl pyrrolidone. A silicone rubber cannot be used in the coating of the implant of U.S. Pat. 4,096,239 since the silicone rubber does not dissolve in body fluids.